1. Field of the Invention
The invention relates generally to processes using sulfuric acid for the extraction/production of alumina for use in the production of aluminum from electrolyte and more specifically to processes which use a basic aluminum alkali sulfate as an intermediate-stage product.
2. Description of the Prior Art
The conventional methods of producing aluminum are not environmentally friendly. The aluminum industry produces in excess of forty million metric tons of green-house gas emissions worldwide each year. The use of so-called "inert anodes" in the production of aluminum has long been recognized as a solution to these emissions, but the high electrolysis temperatures needed, e.g., 950.degree. C., have proven too challenging for commercial production with inert anodes. Such process starts with alumina in the "Bayer process" and uses a caustic as an extractant. The alumina produced from the process embodiment of the present invention can be used at temperatures significantly lower in electrolysis for production of aluminum and therefore reduces the challenge of inert anode use at this reduced temperature.
In order to meet the alumina demand which results from the benefits of operation of electrolytic production of aluminum at 750.degree. C., the industry will need to examine the retrofit capability of technology to current production of alumina as well as new greenfield projects.
This application applies to greenfield projects using low cost raw materials. The use of the process for retrofit of existing production facilities is covered separately as it does not need iron removal steps for the production of "cell grade" alumina.
In many conventional processes, sulfuric acid is used as an extractant. Acid leaching leaves tailing ponds with residual acid and other residues. This environmental consequence has been a significant deterrent to commercial scale production using this method. In other processes, the loss of expensive reagents, yield losses, and reagent-recycling issues have contributed to increased costs of capital and operation. For a description of such methods and issues, see U.S. Pat. No. 4,526,763, which generally dwells on the subject of sodium or potassium metal contaminant in alumina production.